1. Field of the Invention
The present invention relates to a novel technique for the fabrication of a semiconductor device, and more particularly to a method for producing a patterned buried oxide (BOX) film for a semiconductor device.
2. Description of the Related Art
Complementary Metal Oxide Semiconductor (CMOS) circuits, which are fabricated using Silicon On Insulator (SOI) wafers have important advantages over conventional bulk CMOS, such as the absence of latch-up, a reduced parasitic drain-source capacitance, ease of making shallow junctions, and higher immunity to soft-errors (e.g., see J. P. Colinge, Silicon-On-Insulator Technology: materials to VLSI, 2.sup.nd Ed., Kluwer Academic Publishers, 1997).
The use of SOI wafers is not limited to CMOS. Micro-machining applications also use SOI wafers for the fabrication of sensors and thermally isolated detectors. Separation by Implanted Oxygen (SIMOX) is currently the main technique used for the fabrication of Silicon-On-Insulator (SOI) wafers. A SIMOX wafer has a superficial silicon layer on top of a buried oxide (BOX) film.
The buried SiO.sub.2 film (BOX) is formed by implanting oxygen ions beneath the surface of a silicon wafer, and subsequently annealing the wafer at a high temperature (e.g., typical anneal temperatures are from about 1300.degree. C. to about 1400.degree. C.). Uniform and continuous BOX with atomically flat Si and SiO.sub.2 interfaces are formed. During the oxygen implant the wafer is heated typically to about 600.degree. C. to about 650.degree. C. This allows the implant damage to "self anneal" during the implant. Depending on the implant dose the temperature window may be extended from 200 C to about 800 C. For the standard dose (such as 10.sup.18 cm.sup.-2), if the implant is carried out at a temperature which is lower than about 500.degree. C., the top Si layer would amorphize due to the implantation damage, and the Si over layer would remain polycrystalline upon further annealing. Wafer heating is achieved without additional heating if a high-current implanter is used. For low beam current implanters, a combination of external heating and beam current heating is used to achieve the required wafer heating.
Conventional efforts to fabricate a patterned BOX, with varying BOX thicknesses mainly focus on implanting the oxygen through a mask. The BOX layer forms only where the oxygen atoms are either not blocked or partially blocked by the implant mask. However, this technique is disadvantageous for several reasons. For example, there is a high density of defects (e.g., primarily dislocations) formed beneath the mask edge region.
Additionally, from a wafer manufacturing point of view it is more desirable to ship "ready to anneal SIMOX wafers", on which the customer may layout his specific mask, rather than implanting wafers that were shipped from customers with the customer mask for a SIMOX implant.
Moreover, a digitally patterned BOX can be produced with only one implant, since the regions protected by the implant mask will not form any BOX. To obtain an SOI wafer with two BOX thicknesses, a second implant using the complementary mask is required. Thus, the number of steps of such a process is increased, thereby making it cumbersome and complex.
Further, the conventional techniques require two masked implants to obtain a patterned BOX. The alignment of the depth position of the first oxygen implant to the depth position of the second implant is not readily achieved, especially if the buried oxide films are thin. The problem is illustrated in FIGS. 11A and 11B in which FIG. 11A shows a desired BOX and FIG. 11B shows a misaligned BOX. Thus, in the conventional technique, a first BOX is not necessarily aligned with a second BOX.